The present invention relates generally to semiconductor processing and, more particularly, to improved techniques for fabricating gate dielectrics.
As integrated circuits have become smaller and more densely packed, so have the dielectric layers of devices such as field effect transistors and capacitors. With the arrival of ULSI (Ultra Large Scale Integrated circuit) technology and gate dielectrics of less than 15 angstroms (Å) in thickness, the reliability of the dielectric material is of increasing importance.
Nitrogen doping, particularly Remote Plasma Nitridation (RPN), is a known technique for increasing the dielectric constant (and hence the unit capacitance) of silicon oxide dielectrics. An increased dielectric constant, ∈, helps in reducing the amount of leakage current experienced as compared to an undoped silicon dioxide layer having the same capacitance. However, there are several associated drawbacks with conventional methods of plasma nitridation used in conjunction with gate thickness around 15 Å or less. First, a relatively strong dosage of nitrogen (2.0×1015/cm2 or greater) introduced into an oxide layer by RPN causes additional growth of the layer. This may result in unacceptable gate dielectric thicknesses where it desired to maintain the physical thickness around 15-20 Å. In addition, the direct nitridation of a silicon oxide layer typically results in a non-uniform distribution of nitrogen atoms therewithin. As a result, the uneven growth of film at the interface during nitridation affects the overall uniformity of the film thickness.
Although additional unwanted growth of the gate dielectric may be curtailed by decreasing the dosage of the nitrogen atoms introduced during the plasma nitridation process, this comes at the expense of a lower dielectric constant and, thus, a higher leakage current per unit capacitance. A need, therefore, exists for an improved process for plasma nitridation which results in a higher nitrogen dosage in the dielectric, but does not substantially increase the dielectric thickness or sacrifice the uniformity of film thickness.